1. Field of the Invention
This invention generally relates to the fabrication of optical components, and more specifically, to methods and systems for fabricating optical devices so as to have a given property at a defined wavelength. Even more specifically, the invention relates to methods and systems that may be used to provide active, real-time control of thin film deposition processes using a non-contact and non-disruptive measurement procedure.
2. Prior Art
Many optical communication systems rely on wavelength selective components; some examples include optical add/drop filter banks for wavelength multiplexing, optical isolators which prevent backreflection noise in laser transceivers, and bandpass filters which reduce the optical bandwidth of photodiode receivers to minimize noise and interference. All of these components are commonly fabricated using thin film coatings on optical substrates. These optical films can be less than one-quarter of a wavelength thick in the optical communication range (1300-1500 nm infrared). In particular, wavelength multiplexing systems may require only a few parts per million tolerance on the optical components in a link. These techniques also apply to fabrication of optical surface waveguides, switches, and other components. There are certainly other applications which use filters at different wavelengths (for example, spectroscopy and optical microscopy).
The fabrication of conventional optical filters and related components involves precision control of the glass composition both to control impurities and to insure accurate refractive index profiles. High purity materials and well controlled tolerances are important to the manufacturing process. Very pure optical materials are used, so the dominant factor in optical quality of the finished components is the accuracy with which the process tolerances can be controlled common manufacturing methods include chemical vapor deposition (CVD), in which layers of material are successively built up on a substrate. Multilayer filters can also be produced by using this process in successive steps. As an example of the CVD process, submicron silica particles are produced through one or both of the following chemical reactions, carried out at temperatures of around 1800-2000° C.:SiCl4+O2—>SiO2+2Cl2SiCl4+2H2O—>SiO2+4HCL
This deposition produces a high purity silica soot which is then sintered to form optical quality glass. There are several variations on this approach, including modified chemical vapor deposition (MCVD) and plasma-enhanced chemical vapor deposition (PECVD). In both cases, layers of material are successively deposited, controlling the composition at each step, in order to reach the desired film thickness, refractive index profile, or optical transmission and reflection coefficients.
MCVD accomplishes this deposition by application of a heat source, such as a torch, over a small area on the substrate. This heat is necessary for sintering the deposited SiO2 and for the oxidation reactions shown above. Submicron particles are deposited at the leading edge of the heat source; as the heat moves over these particles, they are sintered into a layered, glassy deposit. This requires fairly precise control over the temperature gradients in the tube, but has the advantage of accomplishing the sintering and deposition in one step. The precise control required in this case results in low yields and high manufacturing costs; it also means that the exact optical transfer function (OTF) of the filters cannot be controlled, so there is unavoidable performance degradation when filters made by such a process are paired with arbitrary wavelength optical sources. Nevertheless, MCVD accounts for a large portion of the optical fiber produced today, especially in Europe and America, and this process is the basis of a multi-billion dollar annual market.
By contrast, the PECVD process provides the necessary energy for the chemical reactions by direct radio frequency (RF) excitation of a microwave generated plasma. Since the microwave field can be moved very quickly along the substrate (since it heats the plasma directly, not the substrate itself), it is possible to traverse the substrate thousands of times and deposit very thin layers at each pass. Currently, there is no method for precise control of the layers or OTF at each step of this process, which again results in low yields and higher costs. A separate step is then required for sintering of the glass. In both MCVD and PECVD cases, the substrates often require a final heating to around 2150° C. in a furnace to anneal the substrate and thin films.
In an alternate embodiment, a rotating substrate is used for subsequent CVD; an external torch fed by carrier gasses is used to supply the chemical components for the reaction, as well as to provide the necessary heat for the reaction to occur. Two such processes which have been widely used, particularly in Japan, are the outside vapor deposition (OVD) and the vapor axial deposition (VAD) methods. Much of the control in these techniques lies in the construction of the torch, and hence is not very precise.
For example, OVD is basically a flame hydrolysis process in which the torch consists of discrete holes in a pattern of concentric rings which each provide a different constituent element for the chemical reactions; a stream of oxygen is used between successive rings to act as a shield between the different chemicals. The torch is moved back and forth along the rotating preform and the dopants in the flame are dynamically controlled to generate the desired optical profile. OVD has well documented problems because of its high cost (substrates are limited in size, as it is a batch process) and technical problems such as difficulty in removing all the water (OH groups) from the formed glass and a tendency for the filters to have a large depression in refractive index near the middle.
The VAD process is similar in concept, using a set of concentric annular apertures in the torch; in this case, the preform is pulled slowly across the stationary torch. It has been shown that by mixing dopants into the SiCl4—O2 feed, the proportion of dopant deposited with the silica varies with the temperature of the flame; if a wide flame is used, the temperature gradient produces a graded portion of dopants. This process has also proven to be difficult to control at the precision tolerances required for wavelength multiplexer system components.